(1) Field of the Invention
The present invention relates to a digital loop filter and to a phase-tracking circuit using a digital loop filter. More particularly, it relates to a phase-locked circuit which uses a digital filter comprising a delay circuit, adders, and so on, and which is used, for example, in a phase-comparison radio navigation system such as the Decca (registered trademark) Navigator system.
(2) Description of the Prior Art
A conventional phase-tracking circuit uses an analog phase-locked loop filter comprising an operational amplifier having a high input resistance, a capacitor, resistors, and so on. However, such a conventional phase-tracking circuit has the following disadvantages:
1. The performance of the phase-tracking circuit is affected by a change in environmental conditions, such as temperature, humidity, and so on, because the electrical characteristics of the analog phase-locked loop filter change in accordance with a change in environmental conditions.
2. It is impossible to decrease the size of the phase-tracking circuit when it is used in a low frequency range since in a low frequency range the capacitor used in the phase-locked loop filter must be very large.
3. It is difficult to lower the cost of the phase-tracking circuit when it is used in a low frequency range because the cost of a high performance capacitor is high and it is difficult to form a capacitor having a high performance and a large capacitance in an integrated circuit device.
4. It is impossible to make a phase-tracking circuit which does not require adjustment because the electrical characteristics of the phase-locked loop filter have to be adjusted by using a variable resistor and/or a variable capacitor to compensate for the errors in resistance, capacitance, and so on in the filter circuit.